Known in the art is an exhaust purification system providing air-fuel ratio sensors at an upstream side in a direction of flow of exhaust and at a downstream side in a direction of flow of exhaust from an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine. In such an internal combustion engine, the output of the upstream side air-fuel ratio sensor is used as the basis for feedback control so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio. In addition, the target air-fuel ratio is alternately set to an air-fuel ratio richer than a stoichiometric air-fuel ratio (below, simply referred to as the “rich air-fuel ratio”) and an air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, simply referred to as the “lean air-fuel ratio”) (for example, PLT 1).
In particular, in the internal combustion engine which is described in PLT 1, when an air-fuel ratio corresponding to an output of the downstream side air-fuel ratio sensor (below, also referred to as the “output air-fuel ratio”) becomes a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio or becomes a smaller ratio, the target air-fuel ratio is switched to the lean air-fuel ratio, while when an oxygen storage amount of the exhaust purification catalyst becomes a predetermined switching reference storage amount smaller than the maximum storable oxygen amount or becomes a larger amount, the target air-fuel ratio is switched to the rich air-fuel ratio. According to PLT 1, due to this, it is considered possible to suppress the outflow of NOx from the exhaust purification catalyst.